Time ratio solid state voltage regulator

ABSTRACT

The solid state switches connected to fixed taps on a voltageinjecting transformer winding in a solid state step voltage regulator or load tap changing transformer are controlled synchronously to effectively insert the winding in series with the line for a variable number of half cycles, or preferably full cycles, of a base time interval. In addition to zero and full value voltage insertions, a variable portion of the full value voltage increment is injected in series with the line voltage without increasing the number of switches.

United States Patent Filed Leon J. Goldberg Schenectady, N.Y.

Feb. 2, 1970 Aug. 17, 1971 General Electric Company Inventor App]. No.

Patented Assignee TIME RATIO SOLID STATE VOLTAGE References Cited UNITEDSTATES PATENTS 3,195,038 7/1965 Fry 323/25 3,281,652 10/1966 Perrins323/43.5s

Primary Examiner-J. D. Miller Assistant Examiner-A. D. PellinenAttorneys-John F. Ahern, Paul A. Frank, Julius J.

Zaskalicky, Donald R. Campbell, Frank L. Neuhauser, Oscar B. Waddell andJoseph B. Forman ABSTRACT: The solid state switches connected to fixedtaps on a voltage-injecting transformer winding in a solid state stepvoltage regulator or load tap changing transformer are controlledsynchronously to effectively insert the winding in series with the linefor a variable number of half cycles, or preferably full cycles, of abase time interval. In addition to zero and full value voltageinsertions, a variable portion of the full value voltage increment isinjected in series with the line voltage without increasing thenumber ofswitches.

- lrl/ awe/mamas saw/vs f,; g; c/ecu/r p p2 TIME B4770 ape-M770 anew/7"TIME RATIO SOLID STATE VOLTAGE REGULATOR This invention relates to asolid state step voltage regulator or similar adjustable output voltageequipment having a number of solid state switches to selectively switchvarious combinations of voltage injecting transformer windings intoseries circuit relationship with a load. More particularly, theinvention relates to time ratio operation of the switches to inject allor a portion of the voltage associated with an individual voltageinjecting winding without increasingthe number of switches.

In one form of the solid state step voltage regulator to which theinvention applies, a plurality of digitally related secondarytransformer windings are'connected in and out of the line by means ofsolid state switches connected to each end of the winding and optionallyto taps on the winding. An exciting winding shunting the line energizesthe voltage injecting secondary windings. By rendering conductive theproper switches, the output voltage is maintained approximately constantdespite changes in the input voltage, or is adjusted to a predeterminedvalue. In another form of this type of solidstate step voltageregulator, there is a series transformer in the line, and the solidstate switches are connected to the shunt secondary winding to place allor a section of that winding in series with the series transformerwinding. In either case, the entire voltage-producing winding, or anundivided section of the winding bounded by taps to solid stateswitches, is effectively inserted in series with the line toproduce achange in the output voltage. In view of the present high cost of thesolid state power switches as compared to conventional electromechanicalswitches, there is a need to make the equipment commercially attractive,to reduce the number of solid state switches needed to produce a givennumber of step voltages or, conversely, to increase the number of stepsavailable from a given number of switches.

Accordingly, an object of the invention is to provide an improved solidstate step voltage regulator or other adjustable output voltageequipment in which all or a defined portion of the voltage associatedwith a voltage injecting winding is inserted or removed from the circuitby time ratio operation of the switches connected to the ends of thewinding or winding section.

Another object is the provision of a solid state step voltage regulatorcharacterized by the production of a plurality of selectively availablestep voltages from a single voltage-inject ing winding. or windingsection when averaged over a predetermined time interval.

Yet another object is to provide a greater number of steps in a solidstate step voltage regulator without increasing the number of solidstate switches.

In accordance with the invention, an adjustable output voltage circuitsuch as a solid state step voltage regulator connected between input andoutput terminals includes a plurality of bidirectional conducting solidstate switching means, for example, inverse-parallel pairs of siliconcontrolled rectifiers, and one or more voltage injecting transformerstages each having a shunt exciting winding and an inductively coupledsecondary winding. Coupling means are provided for connecting the solidstate switching means to fixed points or taps on the secondary winding,and for effectively inserting the secondary winding in series with anoutput terminal and removing the winding from the circuit, to therebyselectively inject zero and full value voltage increments synchronouslyin series with the input voltage to regulate or adjust the outputvoltage.

The improvement is made of providing time ratio means for switching theswitches, preferably in the form of a time ratio synchronous gatingcircuit for the solid state switching means. Such a gating circuitincludes means for continuously producing a base time interval that isan integral number of half cycles of the input voltage, and controlmeans for effectively inserting the secondary winding for a variablenumber of half cycles or preferably full cycles of each base timeinterval. In this way a selected portion of the' full value voltageincrement, averaged over the base time interval, is inserted in serieswith the line voltage, i.e., the number of possible step changes isincreased, without increasing the number of switches. Frequently thetime ratio operated voltage injecting transformer stage is used inconjunction with a prior type stage that inserts only the zero and fullvalue voltage increments over the base time interval.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of several preferred embodiments of the invention, asillustrated in the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram of an illustratory form of a solidstate step voltage regulator constructed in accordance with theinvention to include provision for time ratio operation of the solidstate switches;

FIG. 2 is a schematic circuit diagram of a load tap changing transformerwith a different arrangement of windings and switches to which theinvention is also applicable;

' FIGS. 3 a, 3 b, and 3 c are voltagewaveform diagrams showing timeratio operation of the circuit for several different combinations ofbase time intervals and number of integral half cycles conductive duringa base time interval; and

FIG. 4 is a block flow diagram of a control circuit for achieving timeratio operation of the switches connected to one winding in the FIG. 1circuit.

The solid state voltage regulator shown in FIG. 1 comprises a seriesregulator transformer 11 having a secondary winding 1 Is connected inseries circuit relationship with a load 12 and a conventionalelectromechanical line circuit breaker 13 between a pair of inputterminals 14 and 15 that in turn are connected across a source of singlephase alternating current potential. The equipment is ordinarily usedwith the commonly available 60 Hz. source but can be supplied by otherlow frequency power sources. The series regulator transformer 11 injectsvoltage increments into the load circuit to maintain the output voltageapproximately constant within a predetermined range of regulationdespite variations in the line voltage applied to input terminals 14 and15. Regulators of this type are also used to adjust the output voltageto a preselected value. Energy for these voltage increments injected inthe line is derived in a shunt exciting transformer having a primarywinding 16p connected between terminals 14' and 15. Two separatesecondary exciting windings 16a and 16b are each connected by twobidirectional conducting solid state switches, or two pairs ofinverse-parallel unidirectional switches, to the primary winding 11p ofseries regulator transformer 11. Depending on which switch is renderedconductive, the prior art operation of the circuit is that the entirevoltage associated with the voltage-injecting windings 16a or 11612 iseffectively injected in series with the line voltage, to thereby changethe output voltage, or the voltage injecting winding is bypassed and azero voltage increment is injected in series with the line voltage.

In FIG. 1 the solid state voltage insertion and bypass switches areillustrated as being inverse-parallel pairs of silicon-controlledrectifiers. Thyristors Al, A2 and C1, C2 are the respective voltageinsertion switches for windings 16a and 16b whereas thyristors B1, B2and D1, D2 are the bypass switches for the respective windings. Otherpower semiconductors such as the triac and the diac can be used in placeof the pairs of silicon-controlled rectifiers. Both the triac and diacare bilateral thyristors, and the triac is similar to thesilicon-controlled rectifier in that it has a gate electrode forinitiating conduction, whereas the diac is a nongate-type device. Inorder to turn off or commutate the silicon-controlled rectifier andother similar thyristor power semiconductors, it .is necessary to reducethe current through the device to a value below the holding value or tomake the cathode of a device positive relative to the anode for aninterval greater than the turnoff period. The need for specialcommutation circuits is obviated in FIG. 1 by rendering conductive thedevices for complete half cycles of conduction.

For this purpose, a synchronous gating or triggering circuit 17 isprovided to supply gating pulses to the appropriate pair of thyristorsas required within each half cycle. Gating circuit 17 is controlled bythe magnitude and polarity of the error voltage generated by comparing apredetermined reference voltage V and the instantaneous output voltage Yin a summing circuit 18. Suitable gating circuits that can be used aredescribed, for instance, in the Silicon-Controlled Rectifier Manual, 4thEdition, copyright 1967, published by the General Electric Company,Electronics Park, Syracuse, New York. Whenthe load 12 being supplied isa unity power factor load, the gating pulses are supplied and switchingfrom a switch connected to one tap to a switch connected to another tapoccurs at the natural current zero of the line current. A change in theoutput voltage is made at this time by inserting or removing avoltage-injecting winding, by removing the gating pulses from thevoltage insertion switches and supplying gating pulses to the bypassswitches, or vice versa. Gating pulses canbe supplied to both pairs ofsilicon-controlled rectifiers in an inverse-parallel pair, or to onlythat thyristor which would conduct accordingto the circuit condition.When the load 12'being supplied is a nonunity power factor load, thesynchronous transfer of conduction from one pair of switches to anotheroccurs at a time delay (or advance) with respect to the natural currentzero, at the time when the voltage on the voltage-injecting winding ispassing through its zero. Synchronous switching does not occur at thenatural current zero because of the fact that in a voltage regulator thecurrent and voltage in the voltage injecting winding do not have thesame phase relation, and if the switches are inverse-parallelsilicon-controlled rectifiers, there are some phase relations whichwould cause a transfer at the current zero to keep the off-goingthyristor conductive in a circulating path. Therefore, switching issynchronous as related to the circuit transfer from one switch or pairof switches to the other switch or pair of switches, and when used witha nonunity power load is synchronous to a selected delay angle after acurrent zero. In either case, whether the switching circuit transfer isat the current zero or at delay angle after the current zero, a switchor pair of switches connected to a particular tap on a voltage injectingwinding or winding section is conductive for only complete half cyclesof the source alternating current.

There are numerous other possible arrangements of the voltage-injectingwindings and switches, operated to obtain similar results, to which theinvention is also applicable. In FIG. 2 is shown by way of illustrationa load tap changing transformer in which the stages and the connectionin the circuit to make voltage injections is different from FIG. 1. Inthis form of the voltage injecting stages, series regulator transformer11 in the line is not used, and the secondary voltage-injecting windingsinstead are inserted directly in series with the load. Two stages areillustrated, each comprising a centertapped voltage-injecting windinghaving three taps each connected to a bidirectional conducting solidstate switch or a pair of inverse-parallel switches indicated by thesymbol X. EAch stage is therefore capable of producing both additive andsubtractive voltage increments as well as the zero voltage increment.The two pairs of voltage injecting windings 16d, 162 and 16g, 16h areboth magnetically coupled to shunt exciting winding 16s, which is intum'coupled to the primary winding 16p of load transformer 16. Differentvalues of total injected voltage are obtained by making the voltagespans of the stage windings different from one another. For example,

the first stage produces (+1) (1) voltage increments, and

the second stage (+3) (0) (-3) voltage units. Assuming that the entirevoltage associated with a particular winding section is inserted orremoved, according to the prior art teaching, this two-stage regulator,by rendering conductive different combinations of switches A-F, has arange from +4 through zero to *4 injected voltage units in steps of oneunit each.

Thus as theretofore known, a fixed voltage increment is switched in orout of the circuit, for each voltage-injecting winding section provided.The innovation of the present invention is that a variable portion ofthe voltage associated with each voltage-injecting winding or sectionwinding is inserted or removed from the circuit, when averaged over asmall time interval, and this is done without increasing the number ofstep-changing solid state switches. To achieve this mode of operation,the switches associated with a voltage-injecting winding are controlledto connect that winding in the circuit for a fraction of a base timeinterval and to do this repetitively for as many contiguous base timeintervals as desired. Within a base time interval comprising an integralnumber of half cycles, the step-changing switches are renderedconductive for a variable number of half cycles, and the effectiveinjected voltage over the base time interval is determined by the ratioof the number of half cycles of conduction to the number of half cyclesin the base time interval. This is known as time ratio operation orproportional control of the solid state switches. Hence, in FIG. 1 theimprovement is achieved in simple terms by the addition of a time ratiooperation circuit 19 for controlling synchronous gating circuit 17.

The principle of time ratio operation of the switches connected to avoltage injecting winding is illustrated in FIGS. 30- 30 for variouscombinations of number of integral half cycles in the base timeinterval, N, and number of integral half cycles during which theswitches are conductive, n The conductive half cycles are shaded. InFIG. 3a the base time interval is four half cycles and when two halfcycles are conductive as shown, the effective average value of voltageinsertion is one-half the full voltage increment winding value. If theswitches conduct for only one half cycle in each base time interval,then the effective average value of voltage insertion is of courseonequarter of the full value, and is three-quarters of the full value ifthe switch is conductive for three half cycles in each base timeinterval. Consequently, there are a maximum of four values of voltageinsertion for that winding. Another consideration, however, is theavoidance of a DC component in the voltage-injecting transformer. InFIGS. 30 and 3b, N, the number of half cycles in a base time interval,is respectively even and odd, whereas in both cases n, the number ofconductive half cycles, is even. In FIGS. 3a and 317, then, no DCcomponent is produced. In FIG. 3c both N and n are odd, but the DCcomponent in adjacent base time intervals balance one another, thusbeing equivalent to a low frequency AC. This is usually tolerable andnot as undesirable as the case when N is even and n is odd, whichproduces a true DC component assuming that all base time intervals andpatterns of conduction within the intervals are identical. Although a DCcomponent can be tolerated if it is small, as would be true if theamount of voltage regulation were small, in general the preferred formis one without the DC component such as is shown in FIGS. 3a and 3b.These require that the switches which insert the voltage-injectingwinding into the circuit be conductive for pairs of half cycles, so thatonce turned on the switch conducts for one or more complete cycles,rather than half cycles. This operating characteristic is obtained byproviding in the gating control circuit a circuit element which sensespolarity of the current or the rate of change of current as itapproaches the current zero just preceding the desired moment oftransfer from one pair of switches to the other pair of switches.

The permissible length of the base time interval, and of the on-offintervals within a base time interval, depends upon the character of theload being supplied. For example, when regulated voltage is beingsupplied to a large heating load having a thermal time constant of manyseconds, the base time interval and on-off intervals within it can bemany cycles or a few seconds long. For general purpose regulation,however, such as on a utility distribution line, a limiting factor isthat these intervals should be short so as to minimize or eliminateobjectionable light flicker on the load circuits. Studies onincandescent lights have been published that show that light flicker isapproximately unnoticeable for voltage fluctuations of 1 percent if theyoccur faster than about 20 times per second, and are unobjectionable tothe average person if they occur faster than about 13 times per second.Three half cycles conducting three half cycles nonconducting results in.a flicker of 20times persecond, and four half-cycles on-off gives aflicker faster than l3'per second.

' The advantage of the invention in producing a greater:

number of step changes over the base time interval without increasingthe number of switches will be illustrated with regard toEIG. 1. Assumethatvoltage-injecting winding 16a is a4.

percent winding, while winding 16b is a percent winding. Only winding16a is operated according to time ratio principles, and for a base timeinterval of eight half cycles, i.e., N=8, the effective insertion valuesfor full .cycle steps are 0, 0.25, 0.50, 0.75, and' 1.0 of the incrementmaximum. For a4 per:

cent winding, .the effective"voltageinsertions are in 1 percent steps.The worst light flickeroccurs at the conditionof equal 'on-off periodsof the increment, which is four half cycles; each. This produces anacceptable light flicker. Theseparate 5 percent winding 16b is switchedfull on or off to provide. a totalof pulse generator 32 produces gatingsignals for the bypass switches. A

Insummary, the solid state switches connected to one or more. voltageinjection transformer windings in a synchronously operated solidstatestep voltage. regulator or load tap changing transformer are conductivefor a variable number of half cycles, in a base time interval.Consequently, a variable portion'of the full value of the voltageassociated with the voltage injection winding averaged over a selectedbase 9 percent in l-percent steps. When bothwindings are switched fullon or-full off' according to the priorartoperation,

only zero, 4 percent, 5 percent, and g-percentzselectionsare Possible. t

As another example, let all four winding sections in FIG. 2 have amaximum voltage insertion value of +3 or 3 volts, as the case may be. Byoperating the first stage according to time ratio .principles as hereindescribed, with a base time interval of six half cycles andassuming=integral complete cycles of conduction, there are now thepossible voltage selections for: thatvvinding+3 volts to .3 voltsinl-volt steps. The voltage span 'of the entire regulator is then +6 voltsto [-6volts in 1- volt steps. Thiscompares to the} range of :4 volts-forthe example previously given. Foreach example,there is an increase inthe number of possible step changes, when considered over the length of.the base time interval, without increasing. the number of switches.Synchronous gating circuits for thyristors operated-according to timeratio or proportional control principles are known generally in the art.For'furtherinformation, reference may be made for instance'to Chapters 8and 12 of the aforementioned GE: SCR Manual or to U.S. Pat.- Nos.3,381,226 and 3,486,042. Specific time ratio gating circuits preferablyconstructed with solid state components can be designed following theprior art teaching. Reference may also be made to' theTransistor-Manual, 7 th Edition, Copyright 1964, published by theGeneral Electric Company (see especially chapters Sand 7 on logic anddigital circuitry).

FIG. 4'shows a block flow diagram of one possible form of Q the timeratio gating circuitry as applied to FIG. 1 for time ratio operation ofthe switches connected to voltage-injecting winding 16a. it is assumedthat a separate synchronous gating circuit is provided for theswitchesconnected to winding 16b, and that this winding is eitherinserted or removed from the circuit throughout the entire baseinterval. The current at the output'of the voltage regulator is sensedby a suitable current sensor 20 and the current zero is determined by acurrent zero sensor 21,. The current zero signal is delayed by aselected amount by time delay circuit 22 and supplied as an input toamplifier 23. At the same time, the output voltage is sensed by anappropriate voltage sensor .24 and is integrated to an average voltageover the base time interval by an integrator circuit 25. The averageoutput voltage so obtained is compared witha reference voltage suppliedby circuit 26in a differential amplifier 27, and theerror signal is fedto an amplifier 28. A sawtooth wave generator 29 having a period equalto the base time interval is another input to amplifier 28. The outputfrom amplifier 28 is thus an error signal which is constant over eachbase time interval or is derived at the beginning of the base timeinterval.

' Amplifier 23, in addition to being supplied with information as to thetime delay following each current zero as well as the magnitude of theerror voltage, has time ratio logic for determining which of theswitches is to be supplied with gating pulses at the beginning of eachhalf .cycle. A gate pulse generator 30 for the voltage insertionswitches supplies gating signals as need to switches A1, A2. Eachassociated bypass switch is, of

time interval is inserted or removed from the circuit as determined bytime ratio control principles, and the increase instep changes over thebase time interval is made without increasing the number of switches.Byrendering the switches conductive for full .cycles within the basetime interval, rather than half cycles, there is no; DC component in thevoltage injecting transformer. The permissible length of the base timeinterval is dependent upon the nature of the load. The invention is alsoapplicable to other alternating current adjustable output voltageequipment such asan alternator.

While the invention. has been-shown and described with reference toseveral preferred embodiments thereof, it will be by Letters Patentpoints on said voltage-injecting winding and for effectively insertingsaid winding in series with one output terminal and removing saidwinding from the circuit to thereby selectively inject full-value andzero voltage increments in series with the input voltage-to change theoutput voltage, characterized by time ratio means for synchronouslyswitching said bidirectional solid state switching means, said timeratio means including means for continuously producing a base timeinterval that is an integral number of half cycles of the input voltageand further including means for effectively inserting saidvoltage-injecting winding in series with the output terminal for avariable number of half cycles of each base time interval, whereby overthe base time interval a selected portion of the full value voltageincrement is injected in series with the input voltage,

wherein said circuit includes one voltage-injecting stage comprising avoltage-injecting winding and solid state switching means operatedaccording to time ratio principles to inject a variable portion of thefull value voltage increment when averaged over thebase time intervaland another voltage-injecting stage that is operated to selectivelyinject only a full value voltage increment and a zero I value voltageincrement over the entire base interval.

2. A circuit according to claim 1 wherein said adjustable output voltagecircuit is a step voltage regulator constructed w'ithjsolid stateswitches.

3. A circuit according to claim 1 wherein said adjustable output voltagecircuit is a load tap changing transformer constructed with solid stateswitches.

4. In a solid state voltage regulator connected between input and outputterminals including a plurality of bidirectional conducting solid stateswitching means and at least one voltage-injecting transformer stagehaving a shunt exciting winding and an inductively coupled secondarywinding, and coupling means for connecting said solid state switchingmeans to fixed points on said secondary winding and for effectivelyinserting said secondary winding in series with one output terminal andoutput terminal for a variable number of half cycles oi" each base timeinterval,

, wherebyover the base time intervala selected portion of the full valuevoltage increment is injected in series with the input voltage,

said circuit comprising one voltage-injecting transformer stage operatedaccording to time ratio principles to inject a variable portion of thefull value voltage increment when averaged over the base time intervaland another voltage-injecting transformer stage operated to selectivelyinject only a full value voltage increment and a zero value voltageincrement over the entire base time interval.

5. A circuit according to claim 4 wherein said time ratio synchronousgating circuit is operative to effectively insert said secondary windingin the circuit for a variable number of complete cycles of the base timeinterval.

6. An alternating current adjustable output voltage circuit connectedbetween input and output terminals including a voltage-injectingtransformer having a shunt exciting winding and an inductively coupledsecondary winding, a pair of bidirectional conducting solid stateswitches respectively connected to fixed spaced points on said secondarywinding, and coupling means for effectively inserting said secondarywinding in series with one output terminal and for removing saidsecondary winding from the circuit to se le ely inject full value andzero voltage increments "synchronously in series with the input voltageto change thebut'put voltage, characterized by i a time ratiosynchronous gating circuit for said solid state switches including meansfor continuously producing a base time interval that is an integralnumber of half cycles of the input voltage and further including meansfor effectively inserting said secondary winding in series with theoutput terminal for a variable number of half cycles of each base timeinterval,

said time ratio synchronous gating circuit being operative to insert aselected fractional portion of the full value voltage increment inseries with the input voltage when averaged over each base time intervalin addition to the full value and zero voltage increments.

7. A circuit according to claim 6 wherein said coupling means includes aseries transformer having a primary winding connected in series with theoutput terminal and an inductively coupled secondary winding effectivelyconnected in series with said voltage-injecting transformer secondarywinding by operation of said solid state switches.

8. A circuit according to claim 6 wherein said time ratio synchronousgating circuit effectively inserts said voltage-injecting transformersecondary winding in the circuit for a variable number of completecycles of the base time interval.

1. An alternating current adjustable output voltage circuit connectedbetween input and output terminals including a plurality ofbidirectional conducting solid state switching means and at least onevoltage-injecting winding, and coupling means for connecting said solidstate switching means to fixed points on said voltage-injecting windingand for effectively inserting said winding in series with one outputterminal and removing said winding from the circuit to therebyselectively inject full value and zero voltage increments in series withthe input voltage to change the output voltage, characterized by timeratio means for synchronously switching said bidirectional solid stateswitching means, said time ratio means including means for continuouslyproducing a base time interval that is an integral number of half cyclesof the input voltage and further including means for effectivelyinserting said voltageinjecting winding in series with the outputterminal for a variable number of half cycles of each base timeinterval, whereby over the base time interval a selected portion of thefull value voltage increment is injected in series with the inputvoltage, wherein said circuit includes one voltage-injecting stagecomprising a voltage-injecting winding and solid state switching meansoperated according to time ratio principles to inject a variable portionof the full value voltage increment when averaged over the base timeinterval and another voltageinjecting stage that is operated toselectively inject only a full value voltage increment and a zero valuevoltage increment over the entire base interval.
 2. A circuit accordingto claim 1 wherein said adjustable output voltage circuit is a stepvoltage regulator constructed with solid state switches.
 3. A circuitaccording to claim 1 wherein said adjustable output voltage circuit is aload tap changing transformer constructed with solid state switches. 4.In a solid state voltage regulator connected between input and outputterminals including a plurality of bidirectional conducting solid stateswitching means and at least one voltage-injecting transformer stagehaving a shunt exciting winding and an inductively coupled secondarywinding, and coupling means for connecting said solid state switchingmeans to fixed points on said secondary winding and for effectivelyinserting said secondary winding in series with one output terminal andfor removing said secondary winding from the circuit to selectivelyinject full value and zero voltage increments synchronously in serieswith the input voltage to change the output voltage, the improVementwhich comprises a time ratio synchronous gating circuit for said solidstate switching means, said time ratio synchronous gating circuitincluding means for continuously producing a base time interval that isan integral number of half cycles of the input voltage and furtherincluding means for effectively inserting said secondary winding inseries with the output terminal for a variable number of half cycles ofeach base time interval, whereby over the base time interval a selectedportion of the full value voltage increment is injected in series withthe input voltage, said circuit comprising one voltage-injectingtransformer stage operated according to time ratio principles to injecta variable portion of the full value voltage increment when averagedover the base time interval and another voltage-injecting transformerstage operated to selectively inject only a full value voltage incrementand a zero value voltage increment over the entire base time interval.5. A circuit according to claim 4 wherein said time ratio synchronousgating circuit is operative to effectively insert said secondary windingin the circuit for a variable number of complete cycles of the base timeinterval.
 6. An alternating current adjustable output voltage circuitconnected between input and output terminals including avoltage-injecting transformer having a shunt exciting winding and aninductively coupled secondary winding, a pair of bidirectionalconducting solid state switches respectively connected to fixed spacedpoints on said secondary winding, and coupling means for effectivelyinserting said secondary winding in series with one output terminal andfor removing said secondary winding from the circuit to selectivelyinject full value and zero voltage increments synchronously in serieswith the input voltage to change the output voltage, characterized by atime ratio synchronous gating circuit for said solid state switchesincluding means for continuously producing a base time interval that isan integral number of half cycles of the input voltage and furtherincluding means for effectively inserting said secondary winding inseries with the output terminal for a variable number of half cycles ofeach base time interval, said time ratio synchronous gating circuitbeing operative to insert a selected fractional portion of the fullvalue voltage increment in series with the input voltage when averagedover each base time interval in addition to the full value and zerovoltage increments.
 7. A circuit according to claim 6 wherein saidcoupling means includes a series transformer having a primary windingconnected in series with the output terminal and an inductively coupledsecondary winding effectively connected in series with saidvoltage-injecting transformer secondary winding by operation of saidsolid state switches.
 8. A circuit according to claim 6 wherein saidtime ratio synchronous gating circuit effectively inserts saidvoltage-injecting transformer secondary winding in the circuit for avariable number of complete cycles of the base time interval.